In the sorting of radiation emissive ores such as uranium ore or gold ore which contains traces of uranium the ore particles are fed in bulk or discretely in a stream generally on a moving conveyor past one or more radiation detectors which measure the radiation of the particles. The radiation measurement of the individual particles or zones of particles is then compared with a physical characteristic parameter such as mass or volume to differentiate between particles or zones of particles which are above or below a predetermined grade. The particles are then discharged from the conveyor past or into an arrangement for sorting the particles into accept or reject fractions in dependence on their measured grade.
A problem with ore sorting arrangements of the above type is that the radio-active radiation of the particles is difficult to measure accurately while the particles are moving at speed past the or each detector and this is particularly so when dealing with ores such as gold ores which have weak emissions. This problem leads to sorting inefficiencies and has the following principal causes:
(a) The radio-active emissions from the particles are random in the case of a sorter which employs only one radiation detector, may even be missed. This difficulty is, however, reduced somewhat in more modern sorting equipment by the provision of a plurality of radiation detectors which are arranged in series adjacent and along the path of the particles and connected to a computer for averaging the radio-active count from each particle or zone of radio-active particles statistically to reduce counting errors which may arise as a result of random emissions from the particles. (b) Due to the weak nature of the emissions from small quantities of radioactive material in particles such as is found in South African gold ores the measurable emissions which are picked up by the remote radiation counters may be swamped by or close to the level of background radiation in the measuring zone even though the measuring zone of the sorter is heavily lead shielded. The background radiation has in practice been found to emanate principally from radio-active contamination of the lead shielding and natural cosmic radiation which varies from place to place on the earth's surface as well as natural radioactive trace elements such as potassium 40 which can be present in various geological and building materials.
(c) Radio-active interference from particles adjacent the specific particle or zone of particles being measured.